The primary objective of this substudy of USPHS Study 23 is to determine the proportion of patients with HIV-related tuberculosis who have abnormal pharmacokinetics(PK) of isoniazid and rifabutin. Secondary objectives are to determine the risk factors of abnormal PK; to evaluate the correlation between PK and toxicity of the drugs; and to define and correlate phenotypic INH acetylator status with the results of genotypic acetylator data obtained in the parent trial.